US space agency Nasa says one of its unmanned rover vehicles on Mars has drilled into a rock, the first time this has been done by a robot vehicle.

A Nasa scientist said the vehicle, called Spirit, took nearly three hours to drill the 2.7mm (0.1 inch) hole.

The agency hopes it will provide clues about the geological past of Mars.

The drilling is a boost for the Nasa mission - Spirit malfunctioned shortly after it landed last month but resumed its investigations on Friday.

Its sister vehicle, Opportunity, is on the opposite side of the planet and has been sending pictures back of the terrain.

Preliminary evidence suggests Mars was once warmer and wetter.

'Thrilled'

Nasa scientists hailed the drilling of the hole, which is 45.5mm in diameter, as a significant achievement.

"I didn't think that it would cut this deep," Steve Gorevan, the scientist in charge of rock abrasion tools on the rovers, told the AFP news agency.

"In fact, when we saw virtually a complete circle, I was thrilled beyond anything I could have ever dreamed."

The rock, known as Adirondack, is believed to be made of basalt.

Spirit has been troubled by an overloaded memory since 22 January and engineers had to limit the rover's activities as they sought to clear it.

To debug the rover, engineers had to erase and then reformat the vehicle's flash file system.

Glenn Reeves, the Mars Exploration Rover flight software architect, said it was a remarkable effort - especially so since the recovery operation was directed from Earth at a distance of 200m miles (320m km).

Opportunity, meanwhile, has continued to move away from its landing pad towards an intriguing outcrop of rock that appears to contain very thin layers.

The buggy was scheduled to have completed the drive, which is about five metres from the pad, by Saturday.

Nasa scientists want to know if the outcrop is composed of sedimentary rock, possibly laid down through the action of water or wind, or volcanic rock made up from ash fall.